1. Field of the Invention
The invention relates to the general field of the use of pulsed lasers as drilling, boring or cutting tools. In such laser machining an accurately focused laser beam is directed against a workpiece so as to vaporize a portion of that workpiece. The invention provides an automated method for drilling a number of holes sequentially through the same workpiece.
2. Description of the Related Art
It is known to provide pulsed YAG lasers of the kind used for hole drilling with in-line cameras, so that the operator can see on a visual display screen an image of the target area of the workpiece before drilling commences and after drilling is complete. The displayed image is purely for the laser operator's benefit. It has, however, been proposed that the positioning of the nozzle of the laser relative to the target area of the workpiece surface can be accurately controlled by a manual adjustment of the laser nozzle relative to the workpiece until the image displayed on the VDU by the in-line camera becomes sharply in focus. Such a manual adjustment is, however, slow and subjective, and other methods of accurately positioning the laser nozzle are generally preferred. One such method involves moving the nozzle until it touches the workpiece, and then moving backwards by a defined distance. That method requires an electrically conductive workpiece surface, so that an electrical signal can be generated to indicate the moment of touching. Another method of rapidly positioning the nozzle relative to the workpiece prior to drilling involves the measurement of the capacitance of the gap between the nozzle and the workpiece. This method is effective and is normally accurate. It is however susceptible to error if the humidity of the air between the nozzle and the workpiece varies significantly, if the workpiece is ceramic-coated (in which case the capacitance measurement automatically includes that of the ceramic coating over the underlying metal surface of the workpiece) or if the nozzle is presented to the workpiece at a particularly low angle of incidence.
The manual adjustment of the position of the laser relative to the workpiece is extremely slow, and is used for initial set-up only. After that initial set-up, the laser is used to drill a very large number of holes in the workpiece (sometimes in excess of 400 accurately placed holes) without a further check on the accuracy of the nozzle position.
This invention contemplates for the first time a rapid and automatic method of determining an accurate spacing between the nozzle and the workpiece prior to drilling. The invention also provides an accurate check on the size and accuracy of each hole as it is drilled, thus enabling the entire drilling cycle of multiple holes to be automated.